Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example. One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation is collimated into a fan beam or a pencil beam. Radiation transmitted through the object is attenuated to varying degrees by the contents of the luggage. The attenuation of the radiation is a function of the density of the materials through which the radiation beam passes. The attenuated radiation is detected and radiographic images of the contents of the object are generated for inspection. The images show the shape, size and varying densities of the contents.
To obtain additional information about the contents of the luggage, detectors may be provided to detect scattered radiation, as described in U.S. Pat. No. 5,642,394, for example. Systems may combine detection of scattered radiation with the detection of transmitted radiation.
Another technique to enhance the information that may be derived about the material composition of the contents of the objects is to scan the object with radiation beams having two different energy levels. A ratio of the attenuation detected at two energy levels is indicative of the atomic numbers of the material through which the radiation beam passes. Dual energy systems enable better detection of plastic materials and illegal drugs.
One disadvantage of radiographic imaging is that all items within the object in the path of the radiation beam are superimposed on the image. If there are many items in the object, it may be difficult to distinguish among them. The identification of dangerous items is thereby hampered. In addition, the orientation and shape of the items within the object could effect whether they can be identified on a radiograph. Thin sheets of explosive materials may also be difficult to identify on a radiograph, particularly if they are oriented perpendicular to the scanning beam.
Computed tomography (“CT”) enables the reconstruction of the cross-sectional images of luggage contents, facilitating the identification of the items in the luggage. CT images also provide higher resolution, greater image contrast and greater sensitivity to characteristics of the object being scanned, than radiographs. However, reconstruction of CT images of an object requires a large number of scans of the object at a plurality of angles. Conducting a sufficient number of scans for CT reconstruction is time consuming. Depending on the system used, CT imaging of an entire piece of luggage may be too slow for practical use in screening luggage in airports, for example.
In U.S. Pat. No. 5,367,552 (“the '552 patent”), a source of X-ray radiation is provided on one side of an inner surface of a rotating module and a detector array is provided on the opposite side. Luggage is moved through the module incrementally. The module rotates to scan the luggage at a plurality of angles, at each incremental position. The inspection speed may be increased by pre-screening with a line-scan. Then, only suspicious regions identified by the prescreening step are subjected to CT imaging.
U.S. Pat. No. 6,078,642 (“the '642 patent) discloses a CT scanning system for luggage where data processing techniques are used to speed the inspection rate. As in the '552 patent, an X-ray source and a detector array are disposed on opposing sides of a rotating module. The source may emit a pyramidal cone beam of radiation and the detector array may be 2-dimensional. The module rotates as a piece of luggage is continuously moved through the module, providing helical volumetric CT scanning. CT scanning is said to be provided of the entire piece of luggage, without requiring pre-scanning. The source may emit an X-ray beam of two different energy distributions, as well.
While the smuggling of contraband such as guns and explosives onto planes in carry-on bags and in luggage has been a well known, ongoing concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo containers. Only 2%–10% of the 17 million cargo containers brought to the United States by boat are inspected. “Checkpoint terror”, U.S. News and World Report, Feb. 11, 2002, p. 52.
Standard cargo containers are typically 20–50 feet long (6.1–15.2 meters), 8 feet high (2.4 meters) and 6–9 feet wide (1.8–2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term cargo container will generally encompass pallets, as well.
In contrast to the cargo container size ranges, typical airport scanning systems for carry-on bags have tunnel entrances up to about 0.40×0.60 meters. Scanning systems for checked luggage have travel openings that are only slightly larger. Since only bags that fit through the tunnel may be inspected, such systems cannot be used to inspect cargo containers. The low energies used in typical X-ray luggage and bag scanners, described above, are also too low to penetrate through the much larger cargo containers. In addition, many such systems are too slow to economically inspect larger objects, such as cargo containers.
U.S. Pat. No. 6,292,533 B1 discloses a mobile X-ray inspection system for large objects, such as a cargo container carried by a vehicle, that uses an X-ray source of 450 kV. The source is supported on a truck and a pencil beam is generated to vertically scan the vehicle. Detectors, also supported on the truck or a boom extending from the truck, are provided to detect radiation transmitted through and scattered by the contents of the object. In use, a vehicle to be inspected parks alongside the scanning unit on the truck. The source and detectors are moved horizontally by a translation system within the truck to horizontally scan the vehicle. While having sufficient penetration, use of a pencil beam may be too slow to efficiently scan cargo containers. The scan motion is said to be “exceedingly slow” (⅓–⅙ of a mile per hour).
U.S. Pat. No. 5,917,880 discloses an X-ray inspection apparatus that may be used to inspect cargo containers, that uses X-ray radiation of about 8 MeV, collimated into a vertical fan beam to scan a truck carrying the cargo. A first detector array is aligned with the fan beam to detect radiation transmitted through the truck. A second detector array is provided to detect radiation forward scattered through the truck. The truck is moved through the vertical fan beam. Data from both detectors is used to determine the average atomic number of the attenuating material in the truck to identify the material content in the truck. Images indicative of the material content are then prepared. Data provided by the first detector array is also used to form radiographs of the truck. While faster than a pencil beam, a fan beam may still be too slow to efficiently scan large objects at a reasonable rate.
In U.S. Pat. No. 5,638,420, large containers are inspected by a system on a movable frame. A source of a fan beam, a cone beam or a pencil beam of X-ray radiation, such as a linear accelerator with an accelerating potential in the MV range, is mounted on one side of the frame. A detector array is mounted on an opposing side of the frame. The frame may be self-propelled and advances across the length of the container. Radiographic images are generated for analysis by an operator.
Radiographic images of large objects such as cargo containers suffer from the same problems described above with respect to radiographic images of smaller objects such as luggage. U.S. Pat. No. 5,524,133 discloses scanning systems for large objects such as freight in a container or on a vehicle. In one embodiment, two stationary sources of X-ray radiation are provided, each emitting a beam that is collimated into a fan beam. The sources facing adjacent sides of the freight and the fan beams are perpendicular to each other. A stationary detector array is located opposite each source, on opposite sides of the freight, to receive radiation transmitted through the freight. In addition, X-ray radiation of two different energies are emitted by each source. One energy is significantly higher than the other. For example, energies of 1 MeV and 5 or 6 MeV may be used. A ratio of the mean number of X-rays detected at each energy level by the detector array as a whole for each slice or by the individual detectors of the array is determined and compared to a look up table to identify a mean atomic number corresponding to the ratio. The material content of the freight is thereby determined. Three dimensional images based on the ratios of mean atomic number may be reconstructed from the data collected by both detector arrays. The patent states that while the images are coarse, they enable the shapes of certain items to be determined. In combination with the determination of the mean atomic number of the materials in those items, suspicious items may be eliminated or flagged for further inspection.
While three dimensional images based on radiographs are an improvement over radiographs themselves, the high resolution, improved image contrast and the ability to distinguish small differences in characteristics of items within in an object that are provided by CT scanning would be advantageous in the inspection of cargo containers. The CT scanning units used in airports for luggage and the like discussed above are not readily scaleable to the large sizes required to scan cargo containers. For example, to accommodate a cargo container, the rotating modules of the '552 patent or the '642 patent would need to be greatly enlarged. Such large rotating units, carrying both the sources and the detectors, would be very expensive and would be difficult to operate and maintain.
In medical CT scanning, there is a configuration referred to as fourth generation, wherein a source of X-ray radiation rotates completely around a patient in a path of a circle within a larger, stationary circular detector array. Fourth generation CT scanners have been found to be an improvement over earlier generations of scanners where both the source and the detector arrays are moved. Scanning times are shorter and the construction of the scanner is simpler. The arrangements of sources and detectors in medical CT scanners are described in more detail in Seeram, Euclid, Computed Tomography: Physical Principles, Clinical Applications, and Quality Control, Second Edition, W. B. Saunders Company, (2001), pp. 10, 77–81. While only the source is moved completely around the patient, enlargement of such a system to accommodate large objects such as cargo containers would still be difficult.
Despite the various designs for the inspection of large objects such as cargo containers disclosed in the patents discussed above and in other references, much of the inspection of cargo containers is done manually, if at all. “Checkpoint terror”, U.S. News and World Report, Feb. 11, 2002, p. 52. Practical, efficient, non-intrusive radiation scanners for the inspection of large objects, such as cargo containers, are still needed. The ability to perform CT imaging of large objects is needed, as well.